In a radio communication system employing a cellular radio communication method, such as LTE (Long Term Evolution), a mobile station selects, from base stations respectively forming cells, a base station having the most favorable quality of communication with the mobile station. For example, the mobile station performs the process of selecting the base station by using a specified evaluation formula for evaluating the quality of communication.
Further, the mobile station holds the mobility state thereof in a specified storage unit, and corrects the evaluation formula in accordance with the mobility state. The mobility state is an indicator relating to the moving speed of the mobile station, and can be determined on the basis of the number of base stations selected within a specified time. Specifically, the mobility state can be determined in three stages of Normal-mobility state indicating low-speed movement, Medium-mobility state indicating medium-speed movement, and High-mobility state indicating high-speed movement in accordance with and in ascending order of the number of base stations selected within a specified time.
FIG. 12 is a diagram illustrating an example of a mobility state determination method performed by a mobile station. During a communication or standby state, a mobile station X illustrated in an upper part of the drawing selects, as devices to be communicated therewith, two base stations a and b within a specified time ΔT. Further, a mobile station Y illustrated in a middle part of the drawing selects three base stations a, b, and c within the specified time ΔT, and a mobile station Z illustrated in a lower part of the drawing selects four base stations a, b, c, and d within the specified time ΔT. Herein, Normal-mobility state refers to a state in which the number of base stations selected within the specified time is two or less. Further, Medium-mobility state refers to a state in which the number of the selected base stations is three, and High-mobility state refers to a state in which the number of the selected base stations is four or more. In this case, the respective mobility states of the mobile stations X, Y, and Z in FIG. 12 are Normal-mobility state, Medium-mobility state, and High-mobility state, respectively.
Further, the mobile station corrects, in accordance with the mobility state thereof, an evaluation formula for evaluating the communication quality of a cell, and ultimately performs the process of selecting the base station by using the corrected evaluation formula. For example, if the mobility state of the mobile station is High-mobility state indicating high-speed movement, the mobile station corrects the evaluation formula such that the communication quality of a cell serving the mobile station is surpassed by the communication quality of a cell neighboring the serving cell. Thereby, the mobile station is allowed to promptly select a base station forming the neighboring cell.
Herein, in next-generation LTE such as LTE-Advanced, a variety of radio communication systems each including a radio relay station that relays radio communication between a base station and a mobile station have been studied. Particularly, a mobile relay station (or “mobile router”) provided to (mounted on) transportation, such as a bus and a train, to relay radio communication between a base station and a mobile station that moves along with the movement of the transportation has been drawing attention.
Similarly to the mobile station, the above-described mobile relay station has a mobility state selected from Normal-mobility state, Medium-mobility state, and High-mobility state, and selects the base station in accordance with the mobility state. In this case, the device to be communicated with the mobile station is basically the mobile relay station. Therefore, the mobility state of the mobile station is constantly Normal-mobility state.